Asymmetric dinitrogen-coordinated nickel single-atomic sites for efficient CO2 electroreduction

Developing highly efficient, selective and low-overpotential electrocatalysts for carbon dioxide (CO2) reduction is crucial. This study reports an efficient Ni single-atom catalyst coordinated with pyrrolic nitrogen and pyridinic nitrogen for CO2 reduction to carbon monoxide (CO). In flow cell experiments, the catalyst achieves a CO partial current density of 20.1 mA cmgeo−2 at −0.15 V vs. reversible hydrogen electrode (VRHE). It exhibits a high turnover frequency of over 274,000 site−1 h−1 at −1.0 VRHE and maintains high Faradaic efficiency of CO (FECO) exceeding 90% within −0.15 to −0.9 VRHE. Operando synchrotron-based infrared and X-ray absorption spectra, and theoretical calculations reveal that mono CO-adsorbed Ni single sites formed during electrochemical processes contribute to the balance between key intermediates formation and CO desorption, providing insights into the catalyst’s origin of catalytic activity. Overall, this work presents a Ni single-atom catalyst with good selectivity and activity for CO2 reduction while shedding light on its underlying mechanism.


Introduction
There is a lack of references while it requires restructuration. Some typos are found which should be corrected.
Characterization -XPS can be used to give further insights on the asymmetric nitrogen character, since it is a standard technique that allows the differentiation of N-functional groups. Therefore, it is suggested the study by XPS and inclusion of the results in the paper.
-There is a lack of explanation regarding to the determination of the average valence by linear combination fittings by XANES. Please provide more details on this point. In addition, a more quantitative way to report oxidation state changes by XANES relies on the comparison of the position of the rising edge by the inflection point of the 2nd derivative. It is suggested to include a table and the data regarding this parameter.
-Structural changes conclusions by XANES should be revised since they are speculative. Computational methods, such as pre-edge analysis, are proposed to give more solidity to the results. This point is important since previous reports by Yang et al. Nature Energy. 2018. 3. 140-147 report similar XANES spectra.
- Figure S4.B: there is no signal at around 2.6 Å (expected Ni-C) which is presented in the NiPc data. Could the authors suggest an explanation to this? -Further information is required to complete the methods: sample collection and preparation, parameters, etc.

Electrochemistry in a H-cell
-It is suggested to present the electrochemical data in the IUPAC conversion.
-Please include cyclic voltammograms in the supplementary information to complement electrochemical data.
-Labelling experiments are required, as well as for in situ SR-IRAS measurements.
-It is suggested to provided characterization results after electrolysis to confirm the integrity of the catalyst.

Electrochemistry in GDE
-Current density is reported as 1378.3 mAcm-2 at -1 V but at this potential, the FE(CO) drops. What is the integrity of the catalyst at that potential and under these conditions? -Could the authors explain the variation of the electrolyte from KHCO3 to KOH? -Could the authors provide more details regarding to the long-term electrolysis study? In situ-operando XAS -Could the authors suggest further explanations to the almost no differences observed during reaction? If CO coordination occurs (as suggested by SR-IRAS measurements), variations in the XANES spectra (figure S15) should be noticeable, more probable at peak B (1s-4pz transition). Same if H2O coordination is proposed at -0.2 V, differences in the spectra should be expected. Please suggest explanation to these points.
This paper describes the synthesis of single atomic Ni sites with dinitrogen coordination, obtained by thermal treatment of graphitic carbon nitride under Ar and NH3 environment. The catalyst is then used as electrocatalyst for CO2 reduction (CO2RR), placed on carbon paper and tested with H cell and flow cell configuration. A suite of operando techniques is then applied to understand the behaviour of the catalyst during CO2RR. While the performance appear to be appealing, some improvement to the characterisation techniques, processing and interpretation is needed. Queries: • g-C3N4 is notoriously insulating, and it is only widely used for photocatalysis due to photoactivity/conductivity (doi:10.1002/anie.201403375). Could the authors explain if the state of treated g-C3N4 still remains or pretty much carburised? I think it is worth checking the structural and electrical properties of the g-C3N4 and treated NC with and without the metal to see if there is a difference in the N-doped carbon support.
• Figure 1D: and the discussion at page 3: the Ni-N-C EXAFS is rather featureless, which is expected of a highly disordered/defective local structure. It is suggested to change the naming of the EXAFS features because it can be confused with the sub-panel figure 1 numbers. Also, instead of peaks, probably "features" are more appropriate. For feature "A", the proposed non-centrosymmetry of the Ni scatterer will cause 3d-4p mixing, which gives rise to the increased pre-edge feature "A" intensity (see e.g. doi:10.1021/bi900087w). The assessment that the Ni oxidation state is <2+ is supported.
• Performance-wise, the catalyst appears to be excellent at this scale. I'm wondering what the standing of the catalyst is if Au and Ag catalysts are also included in the comparison, as the authors claim in page 4 "Ni-N-C outperformed most reported catalyst under same electrochemical conditions". I'm wondering if energy-efficiency calculation, especially for the flow cell (as it is done with similar conditions with others), have been performed. I think Ag is relevant to compare as it is the incumbent catalyst that has shown large current deployment and much longer stability (see e.g. doi:10.1038/s41565-020-00823-x) • Although some degree of catalytic activity stability is displayed in Figure 2E, a pertinent question to single-atom type catalyst is the inevitable surface reconstruction and agglomeration. With the high current/turnover, I think catalyst migration and agglomeration may be unavoidable. Could the authors comment on this aspect? There are some reports suggesting various operando techniques to track, (for example doi:10.1021/acs.nanolett.0c03475; doi:10.1021/acs.chemrev.2c00495; doi:10.1038/s41929-018-0182-6), I'm wondering if such approach has been attempted (for example, concurrent with the operando XAS shown in Figure 3).
• One follow-up question for the stability: why the flow cell stability is only limited to 18 hours?
• The assignment and interpretation of SR-IRAS peaks are not as unambiguous as expected. Ni-CO (monocarbonyl) is expected around 2118 cm-1 in cited ref 26 but is ascribed to 2030 cm-1 in the manuscript. Perhaps, a comparison with 13CO isotope (and examination of peaks at slightly wider wavenumber range, say 1800 to 2800 cm-1) may reveal more information and give a bit more certainty. Is it possible that the authors are looking at co-adsorption of CO and OH or H? (See e.g. DOI: 10.1016/0039-6028(94)00744-6 and DOI: 10.1016/S0167-2991(96)80058-9). I am also curious if any insitu measurement for the alternative Ni-PC has been done, and if different behaviour of CO adsorption (or co-adsorption) is also seen on Ni-PC.
• XAS, Page 6, "The analysis of XANES spectra at different potentials found that the peaks A and B slightly changed, indicating…" This labelling of "Peaks A and B" are very confusing and is easily mistaken for the earlier IR spectroscopy. The terminology is also confusing, because actually Figure 3C is the R space derivative obtained from data in Figure S15A, which qualifies as EXAFS for the energy range is sufficiently large (8320 to 8420) to cover the oscillations (all Ni XAS shown in this work has the same energy range). The interpretation of the EXAFS data also can be improved. For example, what is the significance of ca. 0.03 A of first neighbour (Ni-N) radial distance change? By the way the correct atomic distances are the ones listed in Table S5. What are the error values for the fitting, and if 0.03 A is within the error? How much elongation should we be expecting in Ni-N when OH or CO is adsorbed on Ni (e.g. can this be corroborated with the calculation)?

Introduction
There is a lack of references while it requires restructuration. Some typos are found which should be corrected. Response: Thanks. We have re-arranged the Introduction and corrected the typos in the revised manuscript.
1.2 Characterization -XPS can be used to give further insights on the asymmetric nitrogen character, since it is a standard technique that allows the differentiation of N-functional groups. Therefore, it is suggested the study by XPS and inclusion of the results in the paper. Response: Thanks. We have provided the N 1s XPS spectra of both Ni-N-C and NC (Fig. R1). It can be observed that the pyridinic and pyrrolic N peaks of Ni-N-C show a slight shift to higher energies, implying the transfer of partial electrons from N to Ni. There findings demonstrate the coordination of Ni with pyridinic and pyrrolic N. We have discussed the implications of these results in the revised manuscript. There is a lack of explanation regarding to the determination of the average valence by linear combination fittings by XANES. Please provide more details on this point. In addition, a more quantitative way to report oxidation state changes by XANES relies on the comparison of the position of the rising edge by the inflection point of the 2nd derivative. It is suggested to include a table and the data regarding this parameter. Response: Thanks. We have provided a thorough explanation and detailed account of our valence state analysis from LCF by XANES in the revised manuscript. To further investigate the valence state of Ni, we have included 1 st derivative data. However, we note that the 2 nd derivative data is insufficient for calculating the valence state of Ni in our work. This is attributed to the low loading of Ni in Ni-N-C, which negatively impacts the quality of the collected XAFS data. Nonetheless, the 1 st and 2 nd derivatives are equal for the valence state analysis (Nat. Commun. 11, 3525 (2020)). To enhance our analysis, we integrate a Table and 1 st derivative figure in the revised manuscript, accompanied by corresponding discussions.   (Fig. R1, R3A,B). Secondly, FT-EXAFS fitting reveals a coordination number of approximately 2.2 for Ni in Ni-N-C (NiPc with Ni-N4) , while N K-edge XANES suggests the maintenance of graphitic N and a decrease in pyrrolic and pyridinic N after Ni incorporation (Fig. R3C,D). As a supplement, pre-edge simulation is carried out. The simulated XANES spectrum of Ni for Ni-N-C is highly consistent with the experimental data, demonstrating the predominant moiety of Ni atom in the optimized catalyst (Fig. R4). These results solidly demonstrate the asymmetric N coordination of Ni in Ni-N-C, supporting the proposed structure as the most likely. Furthermore, our sample is distinct from the Ni single-atom catalysts reported by Yang et al. To facilitate comparison, we include their spectra below, which reveal clear differences between the XAS spectra of the samples (Fig. R5).   1.5 Figure S4.B: there is no signal at around 2.6 Å (expected Ni-C) which is presented in the NiPc data. Could the authors suggest an explanation to this?
Response: Thanks. FT-EXAFS is a technique that results from the modulation of photoelectrons by surrounding atoms at various distances around the element being analyzed. The contributions of long-range scattering paths is typically more significant than those of short-range scattering paths (Chem. Rev. 121, 882-961 (2020)). In the case of NiPc with a welldefined structure, the signal at 2.6 Å arises mainly from the multiple scattering of ligands. In the case of Ni-N-C, the Ni sites are dispersed among the defective carbon planes. The scattered contribution from the second coordination layer is considerably suppressed because of the effects of disorder. This issue has been further elaborated in the revised manuscript.
1.6 Further information is required to complete the methods: sample collection and preparation, parameters, etc. Response: Thanks. We have supplemented the methods with further experimental details in the revised manuscript.
1.7 Electrochemistry in a H-cell -It is suggested to present the electrochemical data in the IUPAC conversion. Response: Thanks. We have revised the data in the revised manuscript with reference to the IUPAC recommendation. A representative legend is shown below (Fig. R6).  To better confirm the intrinsic activity difference between different catalytic sites, the TOF values can be calculated. We have calculated the TOF values at -0.7 VRHE for both catalysts using the same procedure (Table R2). Table R2. Comparison of apparent TOFs of CO generation of Ni-N-C with A-Ni-NS. Sample Ni-N-C A-Ni-NSG TOF at -0.7 VRHE (site -1 h -1 ) ~37300 ~11900 Our results indicate that the Ni sites in Ni-N-C possess better intrinsic activity. In Fig. 2C of main text, the TOF comparison between our catalyst and Yang et al.`s catalyst (A-Ni-NSG) is exhibited. Significantly, our catalyst displays a much higher activity than A-Ni-NSG.
1.10 Labelling experiments are required, as well as for in situ SR-IRAS measurements. Response: Thanks. We have performed 13 CO2 isotope experiment. Based on the experiment with 13 CO2 isotope, we have observed a broad peak ranging from 2255-2395 cm -1 which corresponds to the consumption of CO2 and 13 CO2 (Fig. R8). As the potential was reduced to -0.6 VRHE, we also detected other peaks that correspond to CO adsorption. However, due to the possibility of signal overlap between different CO adsorption peaks, we roughly assigned the detected peaks between 1960-2190 cm -1 . Our findings revealed an isotope shift of approximately 48 cm -1 , which is close to the reported value (ACS Energy Lett. 4, 682-689 (2019). Importantly, these results are consistent with our previous SR-IRAS findings, which strengthens the validity of our research. Isotope data and our discussion on this matter has been included in the revised manuscript. 1.11 It is suggested to provided characterization results after electrolysis to confirm the integrity of the catalyst. Response: Thanks. We have provided characterization results of Ni-N-C after operation at -1.0 VRHE for 10 h. XRD and HAADF-STEM analysis show that Ni-N-C maintains atomic level dispersion within the desired potential range (Fig. R9). Our discussion on this matter has also been included in the revised manuscript. 1.12 Electrochemistry in GDE -Current density is reported as 1378.3 mA cm -2 at -1 V but at this potential, the FE(CO) drops. What is the integrity of the catalyst at that potential and under these conditions? Response: Thanks. The decrease in CO₂ selectivity observed at high potentials can be attributed to the strong competition with hydrogen evolution reaction (HER) at large overpotentials. Furthermore, the supply of CO2 may be insufficient at these high reaction rates, which can be explained by an equilibrium between the thermodynamics of the electrode potential and the diffusive dynamics of the feed gas. Similar observations have been reported in other studies (Nat. Commun. 10, 3602 (2019); Nat. Catal. 2, 1124-1131(2019)).
The catalyst after operation is subjected to XRD and HAADF-STEM analysis, which confirm that the single-atomic structure of Ni-N-C is maintained under these conditions (Fig. R9). This suggests that the major CO2RR activity stems from single Ni sites.
1.13 Could the authors explain the variation of the electrolyte from KHCO3 to KOH? Response: Thanks. The inhibition of the competition of HER is facilitated by the low concentration of protons, corresponding to a high pH value. As a result, we conducted GDE testing in a KOH aqueous solution (ACS Energy Lett. 3, 2527-2532 (2018) ; JACS 143, 3245-3255 (2021)). Further details have been included in the revised manuscript.
1.14 Could the authors provide more details regarding to the long-term electrolysis study? Response: Thanks. To provide a more comprehensive understanding of our experimental procedure, we provide a detailed description of the testing process. Additionally, we have included the complete stability curves in the revised manuscript to supplement our results. During our stability testing in KOH, we observed a sudden decline in the current density of the flow cell device. This drop, however, cannot be solely attributed to the structural deactivation of the catalyst, since the CO selectivity did not show significant changes (Fig. R10A). Instead, we believe that the drop may be due to the salting that is expected in an alkaline environment (JACS 143, 3245-3255 (2021)). To address this issue, we implemented a strategy of regular electrolyte refreshment every 15 hours. With this approach, Ni-N-C in 1.0 M KOH was able to maintain stable for 60 hours even at a current density of ~450 mA cmgeo -2 (Fig. R10B). In neutral conditions, Ni-N-C also exhibited good stability, as demonstrated by its ability to maintain high performance over extended periods of time at current densities of approximately 330 mA cmgeo -2 (GDE ) and 40 mA cmgeo -2 (H cell). Specifically, the FECO of Ni-N-C in all stability tests remains consistently high (>96%).  For atomically dispersed catalysts, each metal site is considered as a catalytic site, while for bulk metal catalysts, the number of active sites is determined by electrochemical methods.
The details of nsite evaluation in the cited literature are introduced below. (3) OD-Au (J. Am. Chem. Soc. 134, 19969-19972 (2012)): The electrochemical surface area of the oxide-derived Au electrode was determined by measuring the charge associated with the stripping of an underpotential deposited Cu monolayer (Fig. S4). It is assumed that the atomic density on the electrochemical surface is that on Au (111) facet and all surface atoms were active sites.
(4) np-Ag (Nat. Commun. 5, 3242 (2014)). The CO partial current density was determined using Fig. 7 in Supplementary Materials, and the electrochemical surface area for per 1 cm 2 of the electrode was calculated as 2650 cm 2 using cyclic voltammetry (Fig. 8 in Supplementary Materials). TOF was determined by assuming Ag (111) facet as the active site.
(5) CoPc-2 (Nat. Commun. 10, 3602 (2019)): The CO partial current density was extracted from Fig. 5a, and active site concentration was obtained from Co II /Co I redox wave from CV under argon atmosphere (Fig. 2).  Fig. 2c and Fig. 3e, respectively. They assumed that the metals on the electrodes were atomically dispersed, and each site was regulated as a catalytic site. Active site concentration was estimated from catalyst loading versus metal concentration. 1.16 In situ-operando XAS -Could the authors suggest further explanations to the almost no differences observed during reaction? If CO coordination occurs (as suggested by SR-IRAS measurements), variations in the XANES spectra (figure S15) should be noticeable, more probable at peak B (1s-4pz transition). Same if H2O coordination is proposed at -0.2 V, differences in the spectra should be expected. Please suggest explanation to these points. Response: Thanks. To improve signal quality during in situ-operando XAFS testing, using high sample loading is necessary. However, it is important to note that only a portion of the catalytic sites react with reactants, which presents a challenge for accurately evaluating reaction processed using XAFS results that provide bulk average information. To overcome these limitations, surface-sensitive SR-IRAS testing was conducted to help understand the reaction mechanism. In addition, detailed XANES spectra have been included in the Supplementary Materials to zoomed-in details of the changes observed (Fig. R11). Discussions have also added to the revised manuscript to improve further clarification. 1.17 Data under inert atmosphere is suggested to be added in order to provide comparison.
In addition, data after electrolysis should be incorporated (coming back to OCP for instance). Response: Thanks. We have incorporated additional XAFS findings in the revised manuscript and extended upon the discussion.  1.18 Suggested to include a zoom insert in figure S15 to clearly observe the variations discussed on text. Response: Thanks. We have provided zoomed-in details of XANES spectra in Fig. S15. Please see Fig. R11 above. Reviewer #2 (Remarks to the Author): In this paper, the asymmetrically coordinated Ni single atom catalyst achieved higher turnover frequency than noble metal nanoparticles. It is noticeable in the CO2RR mechanism that a wellknown poisoning ligand, CO, functions as a promoter for releasing a CO2RR product in a thermodynamically preferable way. Overall, this work will lead the interest of the related research fields after making a few revisions.
2.1. (i) According to the EXAFS description in page 6, line 177-182, and (ii) based on the suggested CO2RR mechanism, the number of coordinated CO molecules should increase from -0.5 V_RHE to -0.8 V_RHE, while the opposite in Fig. 3E. I think the insets for -0.5 V_RHE and -0.8 V_RHE are changed with each other. Response: Thanks. Based on the results from operando experiments, our findings suggest that Ni sites starts to adsorb two CO starting at -0.5VRHE. At more negative potentials, we observed a portion of single CO adsorption, which we attribute to low-valence Ni sites which originate from the very negative voltages induced reduction of some relatively unstable Ni sites in Ni-N-C. This speculation was firstly supported by the 1 st derivative of XANES spectrum collected at -0.8 VRHE, which showed the average valence state of Ni sites was reduced to +1.04 (Fig. R1).
In situ SR-IRAS results of Ni 2+ Pc, which present a peak at approximately 2095 cm -1 further support the above speculation (Fig. R2). To better convey our findings, we modified Figure 3E in the revised manuscript, and its updated version is presented in Fig. R3.    Fig. 4, the authors compare the thermodynamics of CO2RR with and without the second CO molecule, according to the mechanism as *CO2 -> *COOH -> *CO -> * + CO. What about the effect of the second CO on the initial adsorption of CO2 molecule? As starting from bare (*) in the HER thermodynamics of Fig. 4B, the CO2RR in Fig. 4A should be also drawn for a full catalytic cycle. Response: Thanks. Based on Fig. R4, the second CO can create a significant energy barrier during the initial adsorption of the CO2 molecule. We have provided DFT results for the *+CO2 -> *CO2 -> *COOH -> *CO -> * + CO reaction cycle (Fig. R5). We have also incorporated corresponding revisions and discussions in the revised materials.  2.3. It is not necessary to include, but the expression in the line 208-209 can be improved by d-band center theory: "Compared with NiN4, Ni-N2 displayed stronger interaction with molecular orbitals of CO, which obviously altered its electronic structure." It can be understood by the higher d-band center of Ni-N2 that can form stronger metal-CO bond than the lower-lying Ni d states of Ni-N4. Response: Thanks. We have modified the expression. Fig. S17 and S18 should be improved for readers (e.g. resolution, etc.) and there are typos (aborb, unaborb -> adsorb, unadsorb). Please put the (+) signs on the Bader charges in Table S6 for clarity. Response: Thanks. We have modified these figures.
This paper describes the synthesis of single atomic Ni sites with dinitrogen coordination, obtained by thermal treatment of graphitic carbon nitride under Ar and NH3 environment. The catalyst is then used as electrocatalyst for CO2 reduction (CO2RR), placed on carbon paper and tested with H cell and flow cell configuration. A suite of operando techniques is then applied to understand the behaviour of the catalyst during CO2RR. While the performance appear to be appealing, some improvement to the characterisation techniques, processing and interpretation is needed. Queries: 3.1 g-C3N4 is notoriously insulating, and it is only widely used for photocatalysis due to photoactivity/conductivity (doi:10.1002/anie.201403375). Could the authors explain if the state of treated g-C3N4 still remains or pretty much carburised? I think it is worth checking the structural and electrical properties of the g-C3N4 and treated NC with and without the metal to see if there is a difference in the N-doped carbon support. Response: Thanks. We first carried out TEM analysis to analyze the structural changes of g-C3N4 with and without the metal. TEM observation reveals a typical stacking structure for these samples (Fig. R1A,B). Previous work (Angew. Chem. Int. Ed. 51, 9689-9692 (2012)) corroborated that DICY was first assembled to form laid graphic carbon nitrate (g-C3N4) at low temperature, which served as a template to confirm and guide patches of aromatic carbon intermediate (derived from the calcination of glucose) condensation between the interlayer gaps. XPS coupled with elemental mapping analysis reveals the uniform distribution of g-C3N4 in both precursors (Fig. R1E,F and Fig. R2A,B). For Ni-N-C and NC, TEM observation reveals a uniform, wrinkled sheet-like structure (Fig. R1C,D). XPS results indicate that C atoms are primarily sp 2 type, indicating that the precursors have transformed into a N-doped graphene structure at high temperatures (Fig. R2C). This is consistent with our previous work, which shows that the g-C3N4 can be completely decomposed at 750 (Nano Res. 11, 2217-2228 (2018)). Furthermore, N 1s XPS spectra of both Ni-N-C and NC show that they mainly contain graphitic, pyridinic, and pyrrolic N, further evidence the complete transformation of precursors (Fig. R2D). Additionally, it is found that compared with NC, both pyridinic and pyrrolic N peaks of Ni-N-C show a slight shift to higher energies, which indicate the partial electron transfer from N to Ni. There results clearly demonstrated the structural changes of the g-C3N4 and treated NC with and without the metal. We have added corresponding discussions in the revised manuscript. Based on Nyquist plots analysis, it can be observed that all samples show comparable charge transfer resistance. This can be attributed to the formation of an ellectrically conducting carbon layer resulting from glucose carbonization between the g-C3N4 structure (Angew. Chem. Int. Ed. 51, 9689-9692 (2012)), which is further supported by UV-Vis results (Fig. R3,4). Combined with the structure analysis, it can be concluded that the structure and electrical properties of N-doped carbon supports are analogous and not the main factors influencing the performance. The performance is primarily derived from the Ni single-atom sites.    3.2 Figure 1D: and the discussion at page 3: the Ni-N-C EXAFS is rather featureless, which is expected of a highly disordered/defective local structure. It is suggested to change the naming of the EXAFS features because it can be confused with the sub-panel figure 1 numbers. Also, instead of peaks, probably "features" are more appropriate. For feature "A", the proposed non-centrosymmetry of the Ni scatterer will cause 3d-4p mixing, which gives rise to the increased pre-edge feature "A" intensity (see e.g. doi:10.1021/bi900087w). The assessment that the Ni oxidation state is <2+ is supported. Response: Thanks. We have enriched the discussion on Fig. 1D. In addition, we have modified the labeling and description.  (Table R1). Next, we have evaluated the energy efficiency of Ni-N-C in a flow cell and the results are presented in Fig. R6. Our Ni-N-C catalyst exhibits significant efficiency, which is either comparable to or superior to previously reported catalysts.  3.4 Although some degree of catalytic activity stability is displayed in Figure 2E, a pertinent question to single-atom type catalyst is the inevitable surface reconstruction and agglomeration. With the high current/turnover, I think catalyst migration and agglomeration may be unavoidable. Could the authors comment on this aspect? There are some reports suggesting various operando techniques to track, (for example doi:10.1021/acs.nanolett.0c03475; doi:10.1021/acs.chemrev.2c00495; doi:10.1038/s41929-018-0182-6), I'm wondering if such approach has been attempted (for example, concurrent with the operando XAS shown in Figure 3). Response: Thanks. The reconstruction of catalysts in electrochemical reactions has been frequently observed. Our group have reported the catalyst reconstruction in CO2RR before (Adv. Funct. Mater. 30, 2000407 (2020)). However, single-atom catalysts possessing unique isolated active sites are highly active in CO2RR, even at very high current (Science 364, 1091-1094 (2019)). In this work, since the CO2-to CO conversion involve the use and generation of gases, its potential and signal stability are severely impacted, particularly under high current/TOF (iScience 23, 101094 (2020)), making it challenging to use operando methods to study possible changes in the catalyst. Therefore, we conducted post-analysis of the Ni-N-C catalyst after subjecting it to high-current condition. As shown in Fig. R7, the XRD and HAADF-STEM analyses of the post-catalyst reveal that its structure remained single-atomic, and no obvious crystalline formed; this was validated by the absence of a diffraction peak for crystalline structure or cluster/particles. 3.5 One follow-up question for the stability: why the flow cell stability is only limited to 18 hours? Response: Thanks. It is noted that the long-term CO2RR stability of Ni-N-C in 1.0 M KOH is limited due to the severe salt accumulation (JACS 143, 3245-3255 (2021)). To date, almost all CO2RR catalysts can not keep stable for a very long time in KOH. To overcome this issue, a strategy that involves refreshing the electrolyte every 15 hours is explored in this study. The results indicate that Ni-N-C in 1.0 M KOH can function for 60 hours at a current density of approximately 450 mA cmgeo -2 (Fig. R8). Furthermore, the FECO of Ni-N-C in KOH remains consistently high, exceeding >98%. In neutral conditions, Ni-N-C also exhibited good stability, as demonstrated by its ability to maintain high performance over extended periods of time at current densities of approximately 330 mA cmgeo -2 (GDE ) and 40 mA cmgeo -2 (H cell). 3.6 The assignment and interpretation of SR-IRAS peaks are not as unambiguous as expected. Ni-CO (monocarbonyl) is expected around 2118 cm -1 in cited ref 26 but is ascribed to 2030 cm -1 in the manuscript. Perhaps, a comparison with 13 CO isotope (and examination of peaks at slightly wider wavenumber range, say 1800 to 2800 cm -1 ) may reveal more information and give a bit more certainty. Is it possible that the authors are looking at co-adsorption of CO and OH or H? (See e.g. DOI: 10.1016/0039-6028(94)00744-6 and DOI: 10.1016/S0167-2991(96)80058-9). I am also curious if any in-situ measurement for the alternative Ni-PC has been done, and if different behaviour of CO adsorption (or co-adsorption) is also seen on Ni-PC.
Response: Thanks. The peak at approximately 2030 cm -1 is most likely attributed to single CO adsorption over low-valence Ni sites. To investigate this, we conducted a 13 CO isotope experiment. Unfortunately, this experiment did not yield any valuable information, which we suspect was due to the lower CO concentration in the electrolyte compared to CO2. Additionally, the weaker adsorption of CO over Ni-N-C than metals may have contributed to the failure of the isotope experiment. To address this, we instead performed a 13 CO2 isotope experiment. In Fig. R9, a broad peak ranging from 2255-2395 cm -1 which corresponds to the consumption of CO2 and 13 CO2 was observed. As the potential was reduced to -0.6 VRHE, we also detected other peaks that correspond to CO adsorption. However, due to the possibility of signal overlap between different CO adsorption peaks, we roughly assigned the detected peaks between 1960-2190 cm -1 . Our findings revealed an isotope shift of approximately 48 cm -1 , which is close to the reported value (ACS Energy Lett. 4, 682-689 (2019). Importantly, these results are consistent with our previous SR-IRAS findings, which strengthens the validity of our research. Additionally, the presence of low-valence Ni sites can be supported by the 1 st derivative of XANES spectrum collected at -0.8 VRHE, which showed the average valence state of Ni sites was reduced to +1.04 (Fig. R10). It is supposed that low-valence Ni sites originate from the very negative voltages induced reduction of some relatively unstable Ni sites in Ni-N-C. The obtained SR-IRAS spectra of NiPc with a peak at approximately 2095 cm -1 further suggest the above speculation (Fig. R11). While the broad peak made it challenging to distinguish between the adsorption types (single or co-adsorption), since NiPc mainly contains Ni-N4 configuration, we can believe that only single CO adsorption occur over Ni sites in NiPc.
The H adsorption is difficult to detect in neutral conditions due to its low concentration, therefore, no H adsorption was observed in our study. Nonetheless, we did detect broad peaks around 3500 cm -1 (Fig. R12), which we attribute to OH-related adsorption. These peaks gradually decreased with reducing potentials, indicating competitive adsorption between CO2/CO and H2O in a neutral solution. We believe that low-concentration OH is not dominant in adsorption and, as such, emphasize H2O adsorption at -0.2 VRHE in the revised manuscript (Fig. R13).     3.7 XAS, Page 6, "The analysis of XANES spectra at different potentials found that the peaks A and B slightly changed, indicating…" This labeling of "Peaks A and B" are very confusing and is easily mistaken for the earlier IR spectroscopy. The terminology is also confusing, because actually Figure 3C is the R space derivative obtained from data in Figure S15A, which qualifies as EXAFS for the energy range is sufficiently large (8320 to 8420) to cover the oscillations (all Ni XAS shown in this work has the same energy range). The interpretation of the EXAFS data also can be improved. For example, what is the significance of ca. 0.03 A of first neighbour (Ni-N) radial distance change? By the way the correct atomic distances are the ones listed in Table  S5. What are the error values for the fitting, and if 0.03 A is within the error? How much elongation should we be expecting in Ni-N when OH or CO is adsorbed on Ni (e.g. can this be corroborated with the calculation)? Response: Thanks. We have made several modifications to improve the accuracy and interpretation of our XANES and FT-EXAFS data. The peaks have been relabeled, and the terminology has been updated (Fig. R14). Corresponding discussions have been modified. The present consensus is that the accuracy of interatomic distance determination by FT-EXAFS is between 0.01-0.001 Å (Nature 435, 78-81 (2005)), with a difference of 0.03 Å being considered significant. The fitting data recorded in Table S5 has been thoroughly analyzed, taking into account the impacts of phase shift, thus presenting the correct atomic distances. Based on our previous discussion, it should be highlighted that under neutral conditions, H2O adsorption, rather than OH, should be the dominant factor. To further demonstrate the elongation of Ni-N bonds, we have carried out additional calculations as shown in Table R2. It is obvious that the Ni-N bond length after H2O adsorption is almost unchanged, while it increases after CO adsorption. However, H2O adsorption brings Ni-O bond with a long length, while CO adsorption brings a Ni-C bond with short length. Given that XAFS provides an average result, these trends are consistent with in situ XAFS results.   1.2 Similar observation for figure S12. The current density values in S12.B are one order of magnitude lower than the ones in figure S12.A. Please verify. Response: Thanks. Fig. S12B should exhibit the partial current density and Faradaic efficiency of CO (jCO and FECO). Based on the low FECO, jCO in Fig. S12B exhibits very low values during measured potentials. Therefore, to make the figure more comprehensive, we have modified it as Fig. R2. 1.3 In the text is indicated for Fig. S28: "However, when operando testing was performed in Ar-saturated electrolyte, the main peak corresponding to FT-EXAFS shifted significantly to the right only at -0.8 VRHE, which may be caused by the adsorption of oxygen-containing species". This shift is not really evident. Could you please include an arrow or a value to indicate where the significant shift is present? Response: Thanks. We have provided the zoom-in figure to emphasize the shifts in the revised manuscript (Fig. R3). 1.4 Minor: the expression peak a, b and c for the operando SR-IRAS could be modified as writing them in between "" or in bold so the reader can differentiate it from the text avoiding confusion. Response: Thanks. We have written them in bold in the revised manuscript.
Reviewer #2 (Remarks to the Author): The manuscript is now well displaying the comprehensive analyses on the electrochemical performances and theoretical understandings on the material, while some minor corrections are still remaining to be published.
2.2 Clarity in figures (Page 4, Line 103-105) Based on Figure 1D, the ratio of feature III to IV is the maximum in NiPc, which is contradictory with the statement. I think the labels in Figure 1D is just mislabelledred for Ni-N-C and blue for NiPc as done in fig. S6, also based on the consistency between Fig.  1D and fig. S6.
( Figure 4A) Please match the energy values well with the corresponding steps; for example, it would be more readible to color 0.72 eV in green, 0.41eV in blue and 1.15 eV in red, respectively.
( Figure 4C) The cyclic figure is a little bit confusing, since all of the steps involving those where (1, left) no CO adsorbed and (2, right) one CO adsorbed are connected and circulated. Please separate and rearrange them so that they start at different initial structures by clearly showing the initial structures. Otherwise, it is also clear to display the only pathway occuring in NiN2-CO.
( Figure S31) For clarity, the DOSs should be sorted in a systematic way. For example, Ni-unads, Ni-ads, CO-unads, Co-ads; or Ni-unads, CO-unads, Ni-ads, CO-ads. Response: Thanks. We inversed the labeling of Ni-N-C and NiPc in Fig. S6. In the revised manuscript, we have corrected the labeling of Ni-N-C and NiPc in Fig. S6. The corrected figure is shown in Fig. R1 below. Fig. 1D shows that the ratio of feature III to IV of Ni-N-C is higher than that of NiPc, which is consistent with our discussion in the main text. In addition, we have changed the color of energy values in Fig. 4A to enhance its readability (Fig. R2) and Fig. 4C has been divided into two separate figures (Fig. R3). Fig. S31 has been modified systematically as well, and it is presented as Fig. R4.   Response: Thanks. It is not necessary to cite references as these findings are the SR-IRAS results mentioned previously in this work (Fig. 3A). For clarity, we put the corresponding figure in parentheses at the end of the sentence.
2.4 Main text (Page 9, Line 268-269) "This analysis was inconsistent with the experimentally observed excellent activity" >> This text should be now modified or removed, since the aforementioned experimental data already confirmed that "single CO adsorption optimizes the CO2RR" in Page 8, Line 221-224. (Page 9, Line 274-275) Not only this, but the first CO adsorption lowers d-band center of Ni so that the second CO adsorption is less preferred, which means that the second CO can be now released with reduced cost as shown in Figure 4A. Since the main issue is the second CO2 adsorption and CO release, it is better to address this point. (Page 10, Line 280-281) To say 0.72 eV as a significant energy barrier, it is the same with the cost of initial CO2 adsorption in NiN2-CO. Rather, it is enough to say a larger energy barrier than CO emission, NiN2-2CO -> NiN2-CO +CO as indicated in the last step of Figure 4A in NiN2-CO. Response: Thanks. We have modified related discussions in the revised manuscript.
Reviewer #3 (Remarks to the Author): Thank you for the authors for the additional experiments, especially on the isotope studies and additional EXAFS measurement to demonstrate the validity of the data. While most of my questions are answered, I have two follow up questions that will make the article ready for publication.
3.1 I missed the fact that the loading used for EXAFS/XANES measurements are ten times larger than the CO2RR measurement. Also the loading for SEIRAS measurements are different too. Could the authors comment whether these differences in loading will affect the characterisation result? I think in one of the answers to the reviewer the authors have realised the possible difference between bulk technique (e.g. XANES) and surface technique (like XPS and SEIRAS), and one line should be added to acknowledge this possibility. Overall I still agree that if taken together the array of evidence supports the authors proposition. Response: Thanks. In response to the issue of loading used for bulk technique (XAFS) causing signal differences, we agree that this may bring some qualitative findings, however quantitative analysis is difficult. Therefore, to obtain a more sound conclusion, analysis exploring catalytic surfaces, such as SR-IRAS used in our work, is necessary. The influence from the catalyst loading should be smaller in surface-sensitive SR-IRAS than in XAFS. We have added an additional statement to acknowledge this possibility.
3.2 SR-IRAS: I think the SR-IRAS data from Ni-PC is not very conclusive because the single peak wavenumber is closer to peak (a) than the claimed single Ni-CO vibration in Ni-N-C sample (peak C). I agree that this is a tricky issue. What the authors can do is to look for references that demonstrate if Ni-(CO)2 coordination is possible in Ni-PC, and really state the assumption limitation of the argument clearly. Response: Thanks. Based on the newly cited reference, we have made a supplementary discussion to state the assumption of CO single adsorption on NiPc.